Nighttime E-region Research Articles

Characteristics of nighttime E-region over Arecibo: Dependence on solar flux and geomagnetic variations

Electron concentration (Ne) inferred from Incoherent Scatter Radar (ISR) measurements has been used to determine the influence of solar flux and geomagnetic activity in the ionospheric E-region over Arecibo Observatory (AO). The approach is based on the determination of column integrated Ne, referred to as E-region total electron content (ErTEC) between 80 and 150km altitude regions. The results discussed in this work are for the AO nighttime period. The study reveals higher ErTEC values during the low solar flux periods for all the seasons except for summer period. It is found that the E-region column abundance is higher in equinox periods than in the winter for low solar activity conditions. The column integrated Ne during the post-sunset/pre-sunrise periods always exceeds the midnight minima, independent of season or solar activity. This behavior has been attributed to the variations in the coupling processes from the F-region. The response of ErTEC to the geomagnetic variability is also examined for different solar flux conditions and seasons. During high solar flux periods, changes in Kp cause an ErTEC increase in summer and equinox, while producing a negative storm-like effect during the winter. Variations in ErTEC due to geomagnetic activity during low solar flux periods produce maximum variability in the E-region during equinox periods, while resulting in an increase/decrease in ErTEC before local midnight during the winter/summer periods, respectively.

Estimation of nighttime dip-equatorial E-region current density using measurements and models

The existence of the possible ionospheric current during nighttime over low-equatorial latitudes is one of the unresolved issues in ionospheric physics and geomagnetism. A detailed investigation is carried out to estimate the same over Indian longitudes using in situ measurements from Thumba (8.5°N, 76.9°E), empirical plasma drift model (Fejer et al., 2008) and equatorial electrojet model developed by Anandarao (1976). This investigation reveals that the nighttime E-region current densities vary from ∼0.3 to ∼0.7A/km2 during pre-midnight to early morning hours on geomagnetically quiet conditions. The nighttime current densities over the dip equator are estimated using three different methods (discussed in methodology section) and are found to be consistent with one another within the uncertainty limits. Altitude structures in the E-region current densities are also noticed which are shown to be associated with altitudinal structures in the electron densities. The horizontal component of the magnetic field induced by these nighttime ionospheric currents is estimated to vary between ∼2 and ∼6nT during geomagnetically quiet periods. This investigation confirms the existence of nighttime ionospheric current and opens up a possibility of estimating base line value for geomagnetic field fluctuations as observed by ground-based magnetometer.

Daytime E-Region Irregularities During the 22 July 2009 Solar Eclipse over Chung-Li (24.9°N, 121.2°E), a Moderate Mid-Latitude Station

The 22 July 2009 solar eclipse with an obscuration of > 83% at Chung-Li (24.9°N, 121.2°E, Dip 35°N) in Taiwan during noon hours has provided a unique opportunity for us to examine its impact on E-region irregularities which were observed simultaneously by the 52 MHz coherent radar and a co-located ionosonde. Significant observations revealed that the daytime E-region strong backscatter echoes at multiple heights and a sudden intensification of the weak sporadic E-layer during the 22 July 2009 solar eclipse. These results follow the research findings of Patra et al. (2009). As the incident solar radiation suddenly blocked by intruding Moon during solar eclipse events that would generally create night-like ionospheric conditions, it is surmised that the E-region irregularities were indeed induced by the eclipse associated effects. The induced effects resulted in faster recombination of molecular ions, generation of gravity waves and electric fields that could have created a conducive environment to excite plasma irregularities through a gradient-drift instability mechanism. The vertical shears of radar Doppler velocity and the peak radar backscatter at the node of Doppler velocity shear, as resolved by the coherent scatter radar with interferometer technique, were possibly due to the upward propagating gravity waves and wave-induced polarization electric fields. The present observational results should not only be highly useful to ascertain plausible mechanisms responsible for nighttime E-region irregularities, but also provided evidence that a solar eclipse could generate E-region plasma irregularities over temperate mid-latitudes for the first time.

Feasibility of developing an ionospheric E-region electron density storm model using TIMED/SABER measurements

We present a new technique for improving ionospheric models of nighttime E-region electron densities under geomagnetic storm conditions using TIMED/SABER measurements of broadband 4.3 μm limb radiance. The response of E-region electron densities to geomagnetic activity is characterized by SABER-derived NO +(v) 4.3 μm Volume Emission Rates (VER). A storm-time E-region electron density correction factor is defined as the ratio of storm-enhanced NO +(v) VER to a quiet-time climatological average NO +(v) VER, which will be fit to a geomagnetic activity index in a future work. The purpose of this paper is to demonstrate the feasibility of our technique in two ways. One, we compare storm-to-quiet ratios of SABER-derived NO +(v) VER with storm-to-quiet ratios of electron densities measured by Incoherent Scatter Radar. Two, we demonstrate that NO +(v) VER can be parameterized by widely available geomagnetic activity indices. The storm-time correction derived from NO +(v) VER is applicable at high-latitudes.

Mathematical modeling of plasma drifts over equatorial low latitude regions

This paper presents a mathematical model to simulate ionospheric plasma drifts at equatorial low latitude regions by coupling of E- and F-regions. The governing non-linear differential equations (of elliptic and parabolic nature) are solved numerically through finite-difference schemes and obtained neutral winds and electric fields. The temperature and electron density profiles are generated utilizing MSIS-86 atmospheric model. The continuity equation is employed to obtain night-time E-region density profile using measured ionograms at Trivandrum (India). The computed vertical and zonal plasma drifts are comparable with measured Jacamarca plasma drifts with little variations during noon and evening times. The plasma drifts at Trivandrum (8.5° N, 76.5° E, dip 0.5° N) are compared with those of Jicamarca (12° S, 76.9° W, dip 2° N). Neutral wind simulations of present model agree well with those of horizontal wind model (HWM-93). The post-sunset enhancement and its reversal are also discussed.

Influence of solar-geomagnetic disturbances on SABER measurements of 4.3 μm emission and the retrieval of kinetic temperature and carbon dioxide

Thermospheric infrared radiance at 4.3 μm is susceptible to the influence of solar-geomagnetic disturbances. Ionization processes followed by ion-neutral chemical reactions lead to vibrationally excited NO + (i.e., NO +(v)) and subsequent 4.3 μm emission in the ionospheric E-region. Large enhancements of nighttime 4.3 μm emission were observed by the TIMED/SABER instrument during the April 2002 and October–November 2003 solar storms. Global measurements of infrared 4.3 μm emission provide an excellent proxy to observe the nighttime E-region response to auroral dosing and to conduct a detailed study of E-region ion-neutral chemistry and energy transfer mechanisms. Furthermore, we find that photoionization processes followed by ion-neutral reactions during quiescent, daytime conditions increase the NO + concentration enough to introduce biases in the TIMED/SABER operational processing of kinetic temperature and CO 2 data, with the largest effect at summer solstice. In this paper, we discuss solar storm enhancements of 4.3 μm emission observed from SABER and assess the impact of NO +(v) 4.3 μm emission on quiescent, daytime retrievals of Tk/CO 2 from the SABER instrument.

Night-time F-region and daytime E-region ionospheric drifts measured at Udaipur during solar flares

Abstract. Ionospheric drifts measured at a low latitude station, Udaipur (Geomag. Lat. 14.5° N), in the night-time F-region and daytime E-region during solar flares have been studied. The night-time observations, which correspond to the F-region drifts, were carried out on five different nights. The daytime observation corresponding to the E-region drifts is only for one day. It is found that the apparent drift during the solar flare period is reduced considerably, in the daytime E-region as well as in the night-time F-region. The East-West and North-South components of the apparent drift speed are also similarly affected. For the daytime E-region drifts during a flare, increased ionization and subsequent reduction of reflection height is proposed to be the cause of reduced drift speeds. For the night-time F-region drifts, a reduced electric field at the F-region heights resulting from coupling of sunlit and dark hemispheres has been proposed to be the possible cause.Key words. Ionosphere (electric fields and currents; ionospheric disturbances)

Inferred vertical ion velocities associated with intermediate layers

This paper explores one aspect of ion–neutral coupling in the lower thermosphere. Using the continuity equation, we have developed a procedure to estimate the nighttime E-region vertical ion velocities from density observations obtained by the Arecibo Observatory. The technique is viable when adequate signal-to-noise ratios are present (e.g., when intermediate layers are present). Assuming that horizontal neutral winds are primarily responsible for intermediate layer observations, the calculated velocities may be used to infer information regarding the neutral wind field. The variations observed in the calculated vertical ion velocity may result from a variety of sources including fine-scale structures in the neutral wind, electric field effects or the coupling of the two.

Comments on “Modelling the peak of the ionospheric E-layer” by J.E. Titheridge

Q. H. Zhou (zhou@naic.edu)Arecibo ObservatoryArecibo, PR 00612J. D. Mathews (JDMathews@psu.edu)Communications and Space Sciences LaboratoryPennsylvania State UniversityUniversity Park, PA 16802Recently Titheridge (2000) considered a number of nighttime ionization mechanisms including 911-1026 ¯starlight, geocorona and interplanetary background H/He resonant-scattering, and F-region recombinationphoton sources to model the nighttime E-region ionosphere. While his effort is laudable, we do not agree withhis assertion that meteoric input and metallic ions play little role in affecting the mean value of the nighttime E-region peak concentration and altitude. His claim, at least for the temperate zone, does not appear to besupported by the Arecibo incoherent scatter radar observations. Additionally, the argument he used to reach hisconclusion is flawed. We hope the following discussion will help clarify understanding of the nocturnalionization mechanisms.We start the discussion by quoting a paragraph from the Titheridge paper: Ablation of meteors leaves metallicions, largely Mg

Nocturnal high-latitude E-region in winter during extremely quiet conditions

The quiet night-time E-region at high latitudes has been studied using the EISCAT UHF radar. Data from three subsequent nights during a long period of low magnetic activity are shown and typical features of electron density are described. The background electron density is observed to be 5·10 9 m −3 or smaller. Two types of enhancements above this level are observed ; one is due to charged particle precipitation associated with the F-region trough and the other is composed of sporadic- E layers due to waves in the neutral atmosphere. The sporadic- E is observed to exist almost continuously and to exhibit a regular diurnal behaviour. In addition to the typical afternoon and morning sequential layers, a third major descending layer is formed at night after the passage of the F-region trough The afternoon layer disappears simultaneously with the enhancement of the northward trough-associated electric field and the night-time layer appears at high altitudes after the field has again been reduced to a small value. It is suggested that metal ions from low altitudes are swept by the electric field to the upper E-region where they are again compressed to the night-time layer. A set of steeply descending weaker layers, merging to the main night-time layer are also observed. These layers are most probably caused by atmospheric gravity waves. Theoretical profiles for molecular ions indicate that the strongest layers are necessarily composed of metal ions but, during times when the layers are at their weakest, they may be mainly composed of molecular ions.

Zonal irregularity drifts and neutral winds measured near the magnetic equator in Peru

Measurements of zonal irregularity drifts were made by the spaced receiver scintillation and radar interferometer techniques from Huancayo and Jicamarca, respectively. The Fabry-Perot Interferometer operated at Arequipa provided the zonal neutral winds. These simultaneous measurements were performed during evening hours in the presence of equatorial spread- F on three nights in October 1988. The zonal drift of 3-m irregularities obtained with the 50-MHz radar showed considerable variation as a function of altitude. The drift of hundreds of m-scale irregularities obtained by the scintillation technique agreed with the drift of 3-m irregularities when the latter were measured near the F-peak. The neutral winds, on the other hand, sometimes exceeded the irregularity drifts by a factor of two. This is a possible result of the partial reduction of the vertical polarization electric field in the F-region caused by the effects of integrated Pedersen conductivity of the off-equatorial night-time E-region coupled to the F-region at high altitudes above the magnetic equator.

Gradient-drift instability of nighttime mid-latitude Es-layers

We have previously reported observations (Tanaka and Venkateswaran, 1982a,b) which suggest that nighttime E-region irregularities in mid-latitudes are often associated with E s -layers and might arise as a consequence of the cross-field or gradient-drift instability below the layer peak. In this paper we investigate how the ambient ionospheric electric field, the vertical shear (which is assumed to be linear) of the east-west component and the vertical velocity (which is assumed to be constant) of the wind or neutral air motion control the location of the instability, and the apparent drift velocity of the resulting irregularities measurable by ground-based radar.

Optical evidence for mid-latitude charged particle precipitation

Measurements of the N 2 + λ427.8 nm emission from the atmosphere at a mid-latitude station show intensities correlated with Kp, typical values being about 1R at Kp = 3 to 4. It is concluded that charged particle precipitation is a major cause of the emission. Assuming the particles to be electrons of typical auroral energies they make a major contribution to the maintanance of the night-time E-region ionization. On one night the rotational temperature inferred from the N 2 + band shape was approximately 170 K, implying particle penetration to 80–90 km. Attempts to record H β emission from the atmosphere put an upper limit of 0.005 R on the intensity. It is concluded that proton precipitation is not a significant contributor to the E-region ionization.

On the nighttime E-region ionization

In this paper we deal with the ionization sources in the nighttime ionospheric E-region on the basis of experimental data on ion composition. It is shown that [NO +] [O 2 + ] · n e parameter dependence on the solar zenith angle correlates with scattered ultraviolet radiation better than with corpuscular fluxes. It is concluded that scattered radiation is the main ionization source in the nighttime E-region.

Average night-time electron density profiles between 87 and 115 km derived from absorption measurements at 10 fixed frequencies from 250 to 1000 kHz at Tsumeb (19°S)

Results of absorption measurements carried out in 1964 at Tsumeb, S.W.A., at 10 fixed frequencies between 250 and 1000 kHz are presented and interpreted by theory. The calculations yielded the night-time E-region (87–115 km) average electron-density profiles of the background ionization (averages over 14 nights), in which various Es-layers were embedded. The electron density N decreased continuously with time t after sunset in the entire height range. The corresponding recombination coefficients α, defined by the equation dN/ dt = − αN 2, decreased from 9 × 10 −8 cm 3/sec at 87.5 km to 6 × 10 −9 cm 3/sec at 115 km. At heights ranging from 87.5 to 97 km α was proportional to atmospheric pressure. Different recombination coefficients were obtained for the two periods from 1800 to 2400 hr and from 0000 to 0600 hr. All these results suggest that different physical processes occur in the night-time E-region depending not only on height but also on time.

Atmospheric wave-induced instability in the nighttime E-region

Examination of the perturbed continuity equation when the perturbations are the result of an internal atmospheric gravity wave in the E region. The transient response of the ionization is interpreted as the gradient instability and the values of the vertical and horizontal wave numbers that will induce it are plotted for various heights. Only in the presence of westward directed electric fields, which are believed to occur only at night, will the gravity waves induce the gradient instability. Approximate analytic expressions are obtained for the permitted wave numbers as well as for the instability growth times. In the course of this analysis it is shown that in the D region all irregularities, even those that are field-aligned, will tend to move with the ion velocity.

E-region model parameters

The formation of any E-region model is dependent upon a number of key parameters some of which are reviewed in this paper. Particular emphasis is placed on the mean dissociative recombination coefficient of the -region, α M , the most important E-region parameter. It appears that 3 × 10 −7 (300/ T) cm 3 sec −1 is a reasonable value for α M , based on laboratory measurements. Ionospheric studies yield α E , the effective recombination coefficient of the E-region, the deduction of which appears to have suffered from various inadequacies in the past. While normally α E = α M , it is shown how they sometimes differ. Ionization of NO by the direct H Lyman-α flux is significant at twilight, and ionization of NO by scattered H Ly-α is important below about 100 km at night. The E-region above 100 km at night is apparently maintained by H Ly-β. The interrelationships between α E , α M , NO, H Ly-α and H Ly-β during night and twilight are stressed. Electron precipitation is not thought to be a common source of the mid-latitude night-time E-region and it is demonstrated how the presence of a night-time meteor ion layer can lead to erroneous conclusions concerning the source. The influence of transport is briefly discussed as well as some effects of the O O 2 ratio.

Particle influx and the mid-latitude nighttime E-region at solar minimum

Particle influx and the mid-latitude nighttime E-region at solar minimum

Application of incoherent-scatter data to communication

The application of a very sensitive radar system to the observation of signals incoherently scattered in the ionosphere is briefly described. This technique can give more detailed information about height and time variations of bottom-side electron-density profiles than any other method, besides measuring several further parameters of the plasma. Electron-density profiles of the day-time lower Eregion, and of the whole of the night-time Eregion, allow much more reliable estimates than hitherto to be made of nondeviative h.f. absorption, and of the reflection heights of m.f. and l.f. waves. The morphology of the ‘valley’ in the profile between the E and Fregions can be investigated; also the ability to measure details of profile changes during ionospheric disturbances should allow the accompanying deterioration in h.f. propagation conditions to be understood. Incoherent-scatter measurements give unique opportunities of making independent calibrations of statellite instruments in orbit, and of measuring plasma movements induced by neutral aid winds. An empirical law relating the temperature and density of the electrons has been discovered; this will help in predicting the changes in density which should occur when additonal heat energy is deposited in the ionosphere during disturbances.

A sequence of rocket observations of night-time sporadic- E

The variation of the structure of the night-time E-region has been observed in a series of five rocket flights from Wallops Island, Virginia, on 22 February 1968. The launch times were 0009, 0130, 0300, 0430 and 0602 EST. Electron density profiles were obtained by Langmuir probe during rocket ascent. A sporadic- E layer, identified as the intermediate or E2-layer of radio observations, is found in the upper E-region. It is observed at 140 km in the first two profiles and subsequently descends until finally merging with the lower, irregular, region. The layer is initially blanketed by the underlying layers but later appears in ionograms as sequential sporadic- E. Long persistance of layers in the lower E-region is also noted.